he 75th Regiment was raised in 1787 for service in India, and embodied at Stirling in 1788. William Dunsmore served with the 75th as Lieutenant (5 March 1788): Captain (24 May 1794); and Major (8th November 1802). In 1804, he joined the 10th Guards Battalion; in 1805 he fought in the Maratha campaign. He resigned his commission in 1826, as Lt. Colonel.

Dunsmore is portrayed wearing the Seringapatam medal, commemorating what was undoubtedly one of his most glorious campaigns. In 1799, as a Captain with the 75th, and with two Sepoy Regiments and the 74th , under Colonel Hart, Dunsmore captured an important redoubt, from which the 18 pounder battery was able to enfilade much of Tipu's defences The attack was made at 4 o'clock on 17th April 1799, and with great gallantry. 'The enemy were soon compelled to retire, and the height (near the ruined village of Agrarum) being taken possession of perfectly secured our troops against the fire of the fort.' The post was within 1000 yards of the North West angle of the fort, the point at which the final attack took place on 4th May 1799. Here, Dunsmore was one of the 'Forlorn Hope', a small (and often desperate) group of men, nominated or volunteered to be the Advance Party in an attack. The Forlorn Hope were so called because few could hope to survive their brave, almost foolhardy task. At the final attack on Seringapatam, Sergeant Graham of the party only just managed to plant the Union flag on the ramparts before being shot down. Dunsmore survived.

His name does not appear in Beatson's description of the attack, nor in that of Hook, but in Lt Rowley's Journal.

In 1809, the designation of the 75th changed from the 75th Highland Regiment to the 75th Regiment of Foot, no longer clad in tartan. In 1862, they were designated the 75th Stirlingshire Regiment, and in 1881, linked with the Gordon Highlanders as 1st Bn Gordon Highlanders. Their uniform was then altered to full Highland garb, as worn by the 92nd or 2nd Bn Gordon Highlanders. Described by Winston Churchill (1900) as …'the finest Regiment in the world,' the first battle honour in the Gordons proud line is 'Mysore.' Colonel Dunsmore's comrades in the Mysore Wars were fine men from Aberdeenshire and the North East of Scotland. Their story is told at The Gordon Highlanders Museum, in Aberdeen, where the Dunsmore portrait hangs today.